This invention relates to a gear shift control apparatus for use with an automatic transmission having a friction element changeable into an engaged state to effect a change to a lower gear in response to a high level of hydraulic pressure applied thereto.
For example, Japanese Patent Kokai No. 6-11030 discloses a gear shift control apparatus arranged to monitor the gear ratio provided in an automatic transmission to effect a change from a higher gear to a lower gear. The automatic transmission includes a low clutch operable on a working fluid having a changeable pressure. When the gear ratio exceeds a reference value, the working fluid pressure is changed to its high level causing the low clutch to engage so as to produce a change from forth to third gear. This control is referred to as transient hydraulic pressure control. It is proposed, in this Japanese Patent Kokai, to produce such a change down at an appropriate time regardless of the working fluid temperature by increasing the reference gear ratio value as the working fluid temperature increases. The transient hydraulic pressure control is used together with gear shift control made to change the hydraulic pressure to the friction element so as to produce a gear shift according to a predetermined shift schedule.
A plurality of sensors are provided to obtain information on the gear ratio. When at least one of these sensors is subject to failure, the transient hydraulic pressure control is influenced considerably, whereas the gear shift control is not influenced. For this reason, it may be considered to interrupt the transient hydraulic pressure control while continuing the gear shift control in the event of failure of the sensors. Without the transient hydraulic pressure control, however, gear shift operations are repeated at improper hydraulic pressures during transition of a change from fourth to third gear so as to degrade the durability of the friction element.